1. Field of the Invention
This invention relates to a method of producing a composite sheet having a polyolefin-based resin foam layer and an antistatic agent-containing, polyolefin-based resin layer provided on at least one side of the foam layer.
2. Description of Prior Art
Polyethylene foams have been hitherto used as cushioning materials and packaging materials. In particular, antistatic agent-containing polyethylene resin foams have been recognized to be suitable as packaging materials because of their small tendency to cause dust deposition, good softness and excellent cushioning property.
JP-A-H09-169072 discloses an antistatic laminated sheet composed of a polyethylene resin foam layer having a small expansion ratio and a polyolefin layer laminated on the foam layer and containing a bleeding-type antistatic agent. As the antistatic agent, a relatively small molecular weight surfactant such as a glycerin fatty acid ester, a polyoxyethylene alkylamine or an alkyl diethanolamide is used. The laminated sheet exhibits the antistatic property upon absorption of moisture in the air. Thus, under a relatively low humidity condition such as in the winter season, the antistatic property tends to be unsatisfactory. In a high humidity environment, on the other hand, the moisture-absorbed antistatic agent exudes from the sheet and stains its surface so that the surface becomes sticky and whitened. Further, the durability of the laminated foam is unsatisfactory. Namely, when subjected to a wash test using sonication in ethanol, the bleeding-type antistatic agent is washed away and the resulting laminated foam can no longer exhibit desired antistatic properties.
JP-A-2001-347589 discloses a laminated sheet composed of a polyolefin resin foam layer and a conductive non-foamed polyolefin resin layer laminated on the foam layer and containing carbon black. The known laminated sheet has a problem because the carbon black-containing layer, when thin, is apt to be torn and because the color of the sheet is limited to black.
JP-A-2003-136651 discloses a laminated foam composed of a polyolefin resin foam sheet and a synthetic resin layer laminated on the foam sheet and containing a polymeric antistatic agent. The laminated foam has excellent rigidity and can withstand repeated washings without loss of the antistatic property and, therefore, is suitably used as a returnable box. The laminated foam has a relatively large apparent density and is not fully satisfactory for utilization as packaging materials which are required to be soft and to have good cushioning properties.